By the late 1930s,O‘ahu, the capital island of the Territory of Hawai‘i, had become a heavily fortified US military outpost with state-of-the-art shore batteries, airfields, troop formations, fuel depots, even underground command centers, all of it installed and operated by the US Army and its Air Corps to guard the strategic US naval presence at Pearl Harbor, designated a “first-class base” for Pacific Fleet operations in 1919.

But in the wake of Japan’s devastating surprise air attack on Dec. 7, 1941, and with real fear of a full-scale Japanese invasion, defense construction on the island reached new, almost feverish levels. As the war effort peaked in 1943-44, O‘ahu’s military population maxed out at 300,000 personnel. By the time Japan surrendered in 1945, it is said that O‘ahu was the most heavily armed place in the world.

"Fortress O'ahu," they called it.

O'ahu's major military bases cluster in the south-central part of the island and include Pearl Harbor Naval Base, Hickam Air Force Base, Fort Shafter Army Post and Schofield Barracks (including Wheeler Field). Farther afield is Kaneohe Bay Marine Corps Base Hawaii on the eastern coast of the island. The bases, which control 20 percent of O'ahu's land area, contain nine different sites designated National Historic Landmarks, making the island home to the most concentrated collection of active - and historic - military bases in the nation. Furthermore, federal historic-preservation law now ensures that the treasure trove of 20th-century architectural styles represented on the bases will survive.
Yet because of access issues (especially now, in wartime), few civilians know what goes on behind the gates or have any idea how storied many of the sites and buildings are. It's as though several separate walled principalities occupied a big chunk of the island, with 100,000 strangers living in them, with traditional cultures and stories all their own.